This invention relates to method and apparatus for adaptive predictive encoding/decoding of multi-level picture signals, and more particularly to method and apparatus for adaptive predictive encoding/decoding in which a plurality of prediction functions are adaptively switched in accordance with the nature of a picture.
The invention also relates to method and apparatus of motion-compensated interframe encoding and decoding of a television signal for preventing decrease in the encoding/decoding efficiencies caused by scene change.
When predicting multi-level picture signals, typically television picture signals, there are many methods including (1) a case wherein a single prediction function is continuously used, (2) a case wherein a combination of several prediction functions are combination used, and (3) a case wherein a plurality of prediction functions are adaptively switched in accordance with the nature of a picture.
Among types of the last mentioned adaptive predictive method wherein prediction is made by switching a plurality of prediction functions, there are available a method in which a block is formed of a plurality of picture element units, an optimum prediction function is detected in a block unit, and a code representing the optimum prediction function used and a prediction error at that time are transmitted simultaneously, and a method wherein only information corresponding to encoded picture elements is utilized to investigate a local nature of a picture signal for estimating an optimum prediction function of the next picture element. With the latter method, since it is possible to make similar estimation on the receiving side, there is no need of transmitting a code representing the prediction function used, thereby simplifying the circuit construction. This invention relates to the latter type method.
Where only the result of a judgment, at the time of a current picture element, is used for the purpose of estimating an optimum prediction function at the next picture element time, an abnormal prediction error occurs. This error is different from a prediction error obtained from surrounding picture elements. Specifically, a so-called "isolated point" appears which has a large prediction error not withstanding the fact that surrounding prediction errors are small. The isolation point results in errors in the estimation of the optimum prediction functions. Such phenomena are liable to occur in signals on which random noises are superposed. If an optimum prediction function utilized in such a case can be confirmed two-dimensionally, correct estimation would become possible without being strongly affected by the presence of an isolated point.
This will be described with reference to FIGS. 1A and 1B. Let us assume now that two prediction functions I and II are used. Where a picture element signal now being inputted runs on a scanning line displayed on a screen at a time i, it is assumed that the prediction errors pursuant to the prediction function (I) at picture element times i-3, i-2 and i-1 on the scanning line under consideration are 5, 4 and 3, respectively, as shown in FIG. 1A it also is assumed that the prediction errors are 3, 5 and 4 as shown in FIG. B when the prediction function (II) is used. The optimum functions at picture element times i-3, i-2 and i-1 along the present scanning line are functions II, I and I respectively as shown in a row entitled "present" line in FIG. 2. This shows the optimum prediction functions for the encoded picture elements. For each picture element time, one of the two prediction functions (I) and (II) is considered optimum because it is assumed to be attended by a small prediction error.
Since the optimum prediction function at the previous picture element time i-1 is (I), the prediction function X at the present picture element time i would be estimated as (I) if only the result of a judgement on the previous picture element time is used. A matrix of optimum prediction functions at each of several picture element times (i,i+1, i-1, etc.) for each of several picture lines (present, previous, next previous) are shown in FIG. 2. It can be readily understood by examining a pattern of functions (I) and (II) occurring in a vertical direction at the picture element time i that the prediction function (II) is more estimable than the prediction function (I) at the picture element time i.
The phonemenon that the optimum prediction function changes abruptly usually occurs at the contour portions of a picture. Since, at the contour portions, the brightness becomes discontinuous, it is necessary to estimate the optimum prediction function in relation to surrounding conditions. Usually, since a picture contains a large number of contour portions, it is of particular significance to utilize the relation with respect to the surrounding conditions. In the digital transmission of television signals, an interframe encoding system is used wherein a difference signal between adjacent frames (hereinafter called a frame difference signal) is encoded and transmitted to ensure that the number of transmission bits can be greatly reduced as comapred to a based on ordinary pulse code modulation (PCM). Especially, the interframe encoding system permits attainment of a high compression ratio (the ratio of decreasing the transmission bit numbers with respect to PCM) for a still picture or a picture of less movement. In a picture including a large movement, however, the compression ratio decreases due to a large frame difference signal. For the purpose of ensuring a high compression ratio even for a picture including a large movement, a motion compensated interframe encoding system has been proposed. According to this system, a motion of a television signal is detected to generate a prediction signal compensating the motion of the television signal, and the prediction signal is utilized for effective predictive encoding.
FIG. 11 shows an object which was at a point B' (section (a)) in the previous frame has moved to a point A (section (b)) in the present frame. In the motion compensated interframe encoding system, a displacement v (which is termed a motion vector) between point A' at the same position on a televison screen as point A in the present frame and point B' is determined, and as the prediction signal of the signal value Y(r) of point A in the present frame is used the signal value Y'(r+v) instead of the signal value Y'(r) at point A' which is the prediction signal in the case of a simple interframe encoding. In this discussion, r is a position vector showing a position on a television screen.
A prediction error signal Y(r)-Y'(r+v) in the motion-compensated interframe encoding system has a much smaller value than the prediction error signal Y(r)-Y'(r) of the simple interframe encoding system so that with the motion-compensated interframe encoding system, an efficient encoding can be made even for a picture including a large motion.
The method of detecting the motion vector may use a method disclosed in U.S. Pat. No. 4,307,420 to Ninomiya et al, issued Dec. 22, 1981. According to this method, a television signal is divided into a plurality of blocks, the degree of similarity between a television signal of each block in a previous frame at a position displaced by a displacement (which is termed a shift vector relative to a reference at the same position on the television screen) and a television signal of each block in the present frame is evaluated, and a shift vector for the block in the previous frame showing the highest degree of similarity is detected as the motion vector. As the evaluation value for judging the degree of similarity may be used the sum of the absolute values of the differences between signals of the blocks in the present frame and the signals of the blocks in the previous frame shifted by one shift vector or a number of difference signals whose absolute values exceed a predetermined threshold values.
Although the principle and advantage of the motion-compensated interframe encoding system have been described, this system has the following disadvantages.
More particularly, according to the motion compensated encoding system, the number of the transmission bits can be decreased by utilizing a high degree of correlation of television signals between adjacent frames in the same manner as the interframe encoding system. However, when there is no correlation between adjacent frames, for example, at the time of scene change, it becomes impossible to accurately predict the present frame signal from the previous frame signal, thus generating a large amount of information.
In a practical motion-compensated interframe encoding apparatus, the number of the shift vectors can not be infinite and it is inevitable to limit the ranges of movement that can be detected (called a detection range). For this reason, it is impossible to accurately predict the present frame signal from the previous frame signal when a television signal containing motions beyond the detection range is inputted, thus decreasing the encoding efficiency.